The present invention relates to an intensified charge coupled image sensor and particularly to a universal header assembly for such a sensor that permits the sensor to be operated in one of a number of preselected modes.
In prior art intensified charge coupled image sensors, such as the RCA C21205 silicon-intensifier target/charge coupled device (SIT/CCD) camera tube, only an image area or "A" register of the charge coupled device is exposed to the electrons emitted from the photocathode of the intensifier section of the camera tube. A storage area or "B" register and a horizontal output register or "C" register are masked to prevent the electrons from the photocathode from impinging thereon. It is known that in electrostatically focused intensified charge coupled image sensors, such as the RCA C21205 shown in FIG. 1, the lateral magnification of the output image increases from about 1.00 at the center of the image or focal plane (located orthogonal to the optical axis) to about 1.10 at the edge with a nominal value of about 1.03 at 12 mm from the center. This increasing lateral magnification introduces "pincushion" distortion in the image plane that can be minimized and equalized by centering the image or "A" register about the optical axis.
The RCA C21205 SIT/CCD camera tube uses a CCD imager commercially available from RCA Corporation as SID 52501 (formerly 51232) and known as "Big Sid". "Big Sid", represented schematically in FIG. 2, is three-phase operated and has 320 columns and 512 rows (256 in the A register and 256 in the B register) thus providing 81,920 (320.times.256) pixels in the A register for imaging purposes. While not shown in the FIGURES, the imaging area of the CCD is thinned to a thickness of about 8-10 microns.
The operation of the tube of FIG. 1 is well understood. During the so-called integration time, a scene or other image is projected onto an input faceplate of the tube. The light or other incident radiation of the image causes electrons to be emitted, in a pattern corresponding to the intensity of incident radiation, from a photoemissive cathode formed on the interior surface of the faceplate. A potential difference, established between the cathode and a charge coupled device (CCD), accelerates the electrons to a higher energy during the transit from the cathode to the CCD. The emitted electrons are focused by potentials applied to internal image tube components to form an inverted electron image on the imaging register of the CCD. The impinging electrons cause electron-hole pairs to be created at the various locations in the A register in accordance with the electron density reaching the respective locations. The charges, representing picture-element signals, are stored in potential wells under depletion biased electrodes.
Upon the completion of the integration time (during the vertical blanking interval of commercial television), the charge signals which have accumulated (a "field"), are transferred in parallel in the column direction from the A register to the B register by the application of multiple phase voltages. The charges are subsequently transferred, a row at a time, from the B register to the C register, and as each row of charges reaches the C register, it is serially shifted out of the C register in response to the proper shift voltages. The serial shifting of the C register occurs at relatively high speed (during the "line time" of commercial television). During the transfer of the field from the B to the C register, a new field may be integrated in the A register.
In the above-described structure, the CCD is located at the focal plane of the image intensifier tube and disposed substantially orthogonal to the optical axis of the tube. As shown in FIG. 1, the CCD is attached to a metal holder plate having an aperture therethrough that permits electrons emitted from the photocathode to impinge upon the imaging A register of the CCD. The electron impervious body of the holder plate masks the B storage register and the C horizontal output register from electron bombardment which would induce spurious charge in the B and C registers. A ceramic header, shown in elevation in FIG. 3, comprises a ceramic base member and a ceramic plate. The base member has a rectangular aperture slightly larger than the CCD (not shown). The base member aperture is not centered about the optical axis of the tube but is displaced to one side of the axis so that the A register of the CCD, when attached to the holder plate, may be centered about the optical axis. Two columns of metalized contact pads, with 12 pads in each column, are formed on the surface of the base member opposite the surface which is attached to the holder plate. The contact pads extend radially outward from the rectangular aperture and terminate at opposite parallel sides of the base beyond the vacuum envelope of the camera tube. The ceramic plate is fused to the base member of the header structure in such a manner that a portion of the contact pads are disposed between the two ceramic members. The ceramic plate has a substantially circular central aperture with an inside diameter substantially greater than the diameter of the rectangular aperture formed in the base member. The plate thus provides adequate room to electrically connect bonding wires between the 24 CCD electrodes along the periphery of the CCD and the 24 contact pads adjacent the header aperture. The outer edge of the ceramic plate terminates outside the vacuum envelope a distance from the edge of the header thus allowing adequate room to externally contact the radially extending contact pads.
In the above-described structure, the spatial information is limited by the size of the A register to about 82,000 pixels. The spatial information can be increased to about 164,000 pixels by eliminating the B storage register and using both the A and B registers as image registers. A proposed new CCD chip having 457 columns and 572 rows will provide about 234,000 imaging pixels in the combined A and B registers.
In some applications, such as spectroscopy, the entire spatial information capacity of the A and B registers is not required and only a portion of the A and B regisers, for example, an area constituting only a single column of 512 pixels, is used.
Regardless of whether the imaging area comprises the entire A and B registers or only a portion thereof, it is necessary that the imaging area be centered about the optical axis to minimize and equalize the pincushion distortion which is characteristic of an inverter image sensor. To this end, a header assembly is required that permits the sensor to be operated in one of a plurality of possible preselected modes, while masking the nonimaging portion of the CCD from electron emission from the photocathode of the sensor.